Superconductors are materials which can conduct electricity with virtually no resistance when they are cooled below a certain temperature, referred to as the superconductive transition temperature (T.sub.c) For example, pure metals or alloys such as niobium-germanium (Nb.sub.3 Ge) reach the superconductive state when they are cooled below 23K. That degree of cooling requires the use of liquid helium, which condenses at 4K. Liquid helium is expensive and is difficult to manipulate.
A major breakthrough in the commercial development of this technology came in January of 1987, when a yttriumbarium copper oxide ceramic (Y.sub.1.2 Ba.sub.0.8 CuO.sub.4) was prepared which achieved superconductivity at a T of about 90K. See C. W. Chu et al., Phys. Rev. Lett., 58, 405 (1987). This degree of cooling can be readily accomplished with liquid nitrogen (bp 77K or -196.degree. C.), which is much less expensive and easier to handle than is liquid helium.
Since the discovery of high temperature superconductivity in the 2-1-1 oxides (of the form La.sub.2-x Sr.sub.x CuO.sub.4), the 1-2-3 oxides (of the form YBa.sub.2 Cu.sub.3 O.sub.7-x), and the 2-1-2-2 oxides of Bi and Tl (of the form Bi.sub.2 Ca.sub.1+x Sr.sub.2-x Cu.sub.2 O.sub.8+y or Tl.sub.2 Ca.sub.1+x Sr.sub.2-x Cu.sub.2 O.sub.8+y), it has been apparent that the incorporation of these and related materials into existing and new technologies will require the solution of a large number of materials-related problems.
For example, a variety of electronic applications have been proposed for the new superconductors, including high-speed chip interconnections or active devices. See O. K. Kwon et al., IEEE Electron Dev. Lett., 8, 582 (1987); R. Singh et al., in High-T. Superconductivity: Thin Films and Devices, SPIE Pro. Vol. 948; R. B. Van Dover et al., eds. (SPIE-Int'l Soc. for Optical Engineering, Bellingham, Wash., 1988), pp. 3-9; R. B. Laibowitz, MRS Bull., 14, 58 (1989). These applications require high quality thin- and/or thickfilm structures.
There are three requirements to form these high quality superconducting films: stoichiometry, crystallinity, and substrate interaction. The first requirement is to provide the relevant atoms in a precise stoichiometric ratio. Secondly, the correct phase must be formed by proper heat treatment and orientation with the substrate, preferably with the c-axis normal to the substrate surface. Thirdly, chemical reaction between the superconductor and the substrate during annealing must be limited to prevent contamination of the film or changes in stoichiometry by interdiffusion. The most commonly used substrate materials are single crystals of strontium titanate, magnesium oxide and yttria-stabilized zirconia with cubic structures. Films have also been made on other materials such as sapphire, alumina, and silicon.
Of the various film-forming techniques, physical vapor deposition processes such as sputtering [M. Hong et al., Appl. Phys. Lett., 51, 694 (1987)], electron beam evaporation or coevaporation [A. F. Marshall et al., Appl. Phys. Lett., 53 426 (1988)], laser ablation [C. R. Guarnieri et al., Appl. Phys. Lett., 53. 532 (1988)], and spray pyrolysis [D. F. Vaslow et al., Appl. Phys. Lett., 53, 324 (1988); T. F. Kodas et al., Appl. Phys. Lett.. 54, 1923 (1989)] have been extensively investigated. Most of these processes require high vacuum systems and provide very low deposition rates (no more than several hundreds A/min). Another method such as plasma spraying which does not involve vapor deposition but rather melting of particles has also been investigated [J. J. Cuomo et al., Adv. Cer Mat., 2, 422 (1987)], and, recently, chemical vapor deposition (CVD) of superconducting films has been reported by several research groups. See, e.g., A. J. Panson et al., Appl. Phys. Lett., 53, 1756 (1988); P. H. Dickinson et al., J. Appl. Phys., 66, 444 (1989}. In contrast to physical vapor deposition, CVD does not require high vacuum systems and permits a wide variety of processing environments, including low pressure through atmospheric pressure, and is a proven method for depositing compositionally homogeneous films over large areas [M. E. Cowher et al., J. Cryst Growth., 46, 399 (1979)] and even on nonplanar objects. Using CVD, dense, well-crystallized and textured films can usually be obtained by proper control of the deposition conditions. The CVD films can also be grown under an oxygen-rich environment yielding superconducting films without any post-annealing procedure. In a conventional CVD process, reactants are delivered by a cold gas, and chemical reactions take place in a boundary layer over a substrate which is usually heated to promote the deposition reaction and/or provide sufficient mobility of the adatoms to form the desired film structure.
One variant of CVD which is receiving increased interest is plasma-assisted CVD (PACVD) [P. K. Bachman et al., MRS Bull., 13, 52 (1988)]. According to the state of the plasma used, PACVD can be classified into two categories low pressure (non-thermal) PACVD, and thermal PACVD. To date, several research groups have reported on utilizing thermal PACVD to deposit thick coatings of SiC, diamond and the Y-Ba-Cu-O superconductor. A high deposition rate of over 10 .mu.m/min has been demonstrated. In the case of superconducting Y-Ba-Cu-O films as reported by K. Terashima et al., Appl. Phys. Lett., 52, 1274 (1988), a mixture of BaCO.sub.3, Y.sub.2 O.sub.3, and CuO powders (ca. 1 .mu.m) was fed into a thermal rf Ar+O.sub.2 plasma. The generated vapor mixtures were then deposited onto &lt;100&gt; MgO single-crystal substrates placed in the tail flame of the rf plasma. The substrate was heated by the plasma to about 650.degree. C. The deposition rate was more than 10 .mu.m/min. No annealing was performed after the deposition. The structure of the prepared films was identified as the orthorhombic superconducting phase and some of the as-produced films showed the preferred orientation of &lt;001.ltoreq.. The as-deposited film showed a superconducting transition temperature (50% drop of resistivity) of 94K.
However, a continuing need exists for methods to prepare superconducting films.